A boat is travelling at constant velocity across a lake. The boat's engine then increases its thrust force. Explain what happens to the motion of the boat from the moment the thrust increases until the boat reaches a new constant velocity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
When the engine thrust increases the thrust force becomes larger than the resistive drag force. This gives a non‑zero resultant (net) force in the direction of motion. Consequently the boat accelerates and its speed increases. As the speed rises the drag force (which depends on speed) also rises. Eventually the drag again balances the thrust, the resultant force becomes zero and the boat reaches a new constant, higher velocity.
Examiner tips
- Use the word ‘resultant’ or ‘net’ force to show understanding of forces. Show the sequence: thrust > drag → net force → acceleration → increasing drag → thrust = drag → new constant speed. Mention that acceleration is in the direction of the net force.
- Include the fact that the new velocity is higher than the original one, as the thrust is greater than before.
Common mistakes
- Saying the boat simply ‘speeds up’ without linking to net force and acceleration. Forgetting that the drag increases with speed. Claiming the boat never reaches a new constant speed or that the new speed is the same as the old one.
Mark scheme (5 marks)
- When thrust increases, the thrust force is greater than the drag (resistive) force
- There is a resultant (net) force in the direction of motion
- The boat accelerates (speed/velocity increases)
- As speed increases, drag force increases
- The boat reaches a new constant (higher) velocity when thrust equals drag again / resultant force is zero
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Mechanics questions
- A car is travelling along a straight, level road at a constant velocity of 20 m/…
- A person stands on a set of bathroom scales inside a lift. The scales show a rea…
- A student pushes a heavy box across a rough horizontal floor at a constant veloc…
- A shopping trolley is pushed from rest along a flat supermarket car park. The pe…
- A rocket is launched vertically upwards from the ground. At the moment of launch…
- A tennis ball is dropped from rest and falls vertically downward. It hits the gr…